TOI-6508 b: A massive transiting brown dwarf orbiting a low-mass star

Brown dwarfs (BDs) are traditionally defined as objects between giant planets ($\sim$13 $M_{\rm Jup}$) and stars ($\sim$80 $M_{\rm Jup}$), with radii ranging from 0.7 to 1.4 $R_{\rm Jup}$. The lower limit that separates giant planets from BDs corresponds to the ignition of deuterium fusion in the core of the BD. This limit varies within the range 11–16 $M_{\rm Jup}$ depending on the abundance of deuterium and the bulk metallicity (Spiegel et al. 2011). The upper limit that separates the BDs and stars corresponds to hydrogen fusion, and varies within the range 75–80 $M_{\rm Jup}$ depending on the stellar initial formation conditions (Baraffe et al. 2002). Based on this mass definition, the cores of BDs are partially degenerate. To improve the characterization and classification of the BDs, the transit method is very useful as it brings additional information, specifically the radius. However, any candidate companion close in size to $1R_{\rm J}$ can be a brown dwarf, giant planet, or a low-mass star as it is not clear solely from the radius, making the measurement of the companion’s mass crucial for BD discovery.

Transiting BDs orbiting low-mass stars offer us valuable opportunities to measure the radius and mass (with the combination with radial velocity technique) and orbital parameters of the system. The relatively small size of the star leads to a large transit signal on the order of ten percent. Furthermore, the relatively low mass results in a huge radial-velocity signal on the order of several kilometers per second. This makes possible high-precision measurements of a BD’s mass and radius. The mass and radius are key for exploring the physical properties of the BDs, in order to improve our understanding of the mechanisms of formation and evolution of these mysterious sub-stellar objects. (Baraffe et al. 2002; Saumon & Marley 2008; Phillips et al. 2020; Chabrier et al. 2023).

Our current understanding of planetary formation predicts a low probability for the existence of Jupiter-like planets and BDs around low-mass stars with $M_{\star}\leq 0.4M_{\odot}$ , and their formation by core accretion becomes increasingly unlikely as $M_{\star}>0.4M_{\odot}$. (Kanodia et al. 2022; Palle et al. 2021; Burn et al. 2021). However, we have discovered only 9 transiting BDs orbiting low-mass stars to date, with host masses ranging from 0.25–0.65 M_⊙. Due to the small size of this sample, the occurrence rate of BDs around low-mass stars is still highly uncertain; more detections are necessary to compare observations to theoretical expectations.

In this work, we present a new system orbiting a low-mass M dwarf ($M_{\star}=0.17\pm 0.02M_{\odot}$) in a 19-day eccentric orbit ($e=0.28^{+0.09}_{-0.08}$), TOI-6508. This system contains a transiting BD, TOI-6508 b with a mass of $M_{\rm BD}=72.5\leavevmode\nobreak\ M_{\rm Jup}$ and a radius of $R_{\rm BD}=1.03\leavevmode\nobreak\ R_{\rm Jup}$ around an M5 star.

The paper is organized as follows. We present TESS data and ground-based photometric and spectroscopic observations in Section 2. Stellar characterization of TOI-6508 (spectroscopic and spectral energy distribution analysis) is presented in Section 3. Section 4 describes the global modeling of photometric and radial velocity data. Finally, a discussion and our conclusions are presented in Section 5.

We performed a global modeling of transit light curves obtained from the TESS mission (described Section 2.1), and SPECULOOS-South-1.0m and LCO-SAAO-1.0m telescopes (described in Section 2.2), together with the radial velocity measurements collected by the ESO-3.6m/NIRPS spectrograph (described in Section 2.3.1), using the Metropolis-Hastings (Metropolis et al. 1953; Hastings 1970) method implemented in TRAFIT, a revised version of the Markov chain Monte Carlo (MCMC) code described in Gillon et al. (2010, 2012, 2014). We followed the same strategy as described in Barkaoui et al. (2023, 2024). The photometric data were modeled using the Mandel & Agol (2002) quadratic limb-darkening model, multiplied by a transit baseline, in order to correct for external systematic effects related to the time and FWHM of the PSF as well as the airmass, and background light level. The radial velocity data were modeled with the two-body Keplerian model (Murray & Correia 2010).

The baseline model for each transit was selected by minimizing the Bayesian information criterion (BIC; Schwarz (1978)). The error bars of TOI-6508 radial velocity measurements were quadratically rescaled using the ‘jitter’ noise, while the photometric error bars were rescaled using the correction factor $CF=\beta_{w}\times\beta_{r}$, where $\beta_{r}$ is the red noise and $\beta_{w}$ is the white noise (Gillon et al. 2012).

For the joint fit, the global free parameters for transit modeling are the mid-transit time at a reference epoch ($T_{0}$), the orbital period ($P$), the transit depth ($dF$), the impact parameter ($b$), the stellar density $\rho_{\star}$, as well as the total transit duration ($W$). We applied a Gaussian prior distribution on the stellar quadratic limb-darkening coefficients ($u_{1}$ and $u_{2}$), stellar mass $M_{\star}$, radius $R_{\star}$ and effective temperature $T_{\rm eff}$ (computed from SED analysis), as well as the stellar atmospheric parameters computed from spectroscopic analysis (metallicity $T_{\rm eff}$ and surface gravity $\log g_{\star}$). The quadratic limb-darkening coefficients $u_{1}$ and $u_{2}$ for the TESS, Pan-STARRS-$z_{\mathrm{s}}$, Sloan-$i^{\prime}$, Sloan-$r^{\prime}$, and Johnson-$V$ filters were computed using the stellar parameters ($T_{\rm eff}$, $[Fe/H]$ and $\log g_{\star}$) and Tables from Claret et al. (2012); Claret (2018). During our analysis, we converted the quadratic limb-darkening coefficients $u_{1}$ and $u_{2}$ into the combination $q_{1}=(u_{1}+u_{2})^{2}$ and $q_{2}=0.5u_{1}(u_{1}+u_{2})^{-1}$ proposed by Kipping (2013).

We performed two MCMC fits. The first fit assumed an eccentric orbit (i.e. free eccentricity) and the second assumed a perfectly circular orbit (i.e., $e=0$). Our results favored an eccentric orbit solution based on the Bayes factor $BC=\exp{(-\Delta BIC/2)}>1000$. For each transit, a preliminary analysis was performed composed of one Markov chain with $5\times 10^{5}$ steps to compute the $CF$ (correction factor; Gillon et al. (2012)). Then, we performed a final MCMC fit composed of five Markov chains with one million steps to infer the physical properties of the system. The convergence for each Markov chain has been checked based on the Gelman & Rubin (1992) statistical test. Our final results for the eccentric orbit solution are presented in Table 5.

Transiting BDs around M dwarf stars are rare, and they are helpful for understanding the formation and evolution of such systems. Only $\sim 10$ M dwarf/BD systems are known, and more detections are required to probe the formation and evolution paths.

In this paper, we present a BD orbiting a low-mass star, TOI-6508 b. The target was observed with the TESS mission during Sectors 10, 37 and 73 with long-cadence of 1800 s, 600 s and 200 s, respectively (Section 2.1). The candidate was first identified by TESS. Ground-based photometric follow-up observations were performed with the SPECULOOS-South-1.0m and LCOGT-McD-1.0m telescopes (Section 2.2). Radial velocity measurements were collected using the NIRPS spectrograph as described in Section 2.3.1. The host star was characterized by combining optical spectra collected by IRTF/SpeX and Shane/Kast instruments, the spectral energy distribution (SED), and stellar evolutionary models (Section 3). TOI-6508 is a $K_{\rm mag}=11.5$ M5.5 sub-solar star with metallicity of $[Fe/H]=-0.22\pm 0.08$, a mass of $M_{\star}=0.174\pm 0.004\leavevmode\nobreak\ M_{\odot}$, a radius of $R_{\star}=0.205\pm 0.006\leavevmode\nobreak\ R_{\odot}$ and an effective temperature of $T_{\rm eff}=2930\pm 70\leavevmode\nobreak\ K$.

We performed a global analysis of the TESS observations together with ground-based photometric and radial velocity observations in order to derive the physical parameters of the system (Section 4). Table 7 shows the stellar physical characteristics of the host star TOI-6508. The derived physical parameters of the system are presented in Table 5. The posterior distribution parameters of TOI-6508 b are presented in Figure 18. We find that TOI-6508 b is a massive brown dwarf with a mass of $M_{\rm BD}=72.53^{+7.61}_{-5.09}M_{\rm Jup}$ and a radius of $R_{\rm BD}=1.026^{+0.031}_{-0.032}R_{\rm Jup}$. It is the second highest mass ratio BD transiting a low-mass star.

During our modeling, we performed two MCMC fits. The first assuming a circular orbit and the second assuming an eccentric orbit. The best solution is compatible with an eccentric orbit, based on the Bayes factor $BC$. TOI-6508 b orbits its host star with an orbital period of $P=18.99265922^{+0.0000688}_{-0.0000681}$ days and an eccentricity of $e=0.28^{+0.09}_{-0.08}$. Figure 12 shows the posterior probability distribution of the orbital eccentricity and the mass of TOI-6508 b, including the evolutionary models from Baraffe et al. (2003). Additional observations of radial velocity are required to improve the orbital eccentricity and mass measurements of TOI-6508 b (see Figure 13). Figure 14 presents the mass ratio $M_{\rm BD}/M_{\star}$ as a function of the BD mass. TOI-6508 b has the second highest mass ratio among all known transiting BDs.

The surface gravity of the transiting BDs $g_{\rm BD}$, can be derived directly from the transit observations and radial velocity measurements, and is given by,

where, the semi-amplitude $K_{\rm RV}$ and the orbital eccentricity $e$ are derived from the radial velocity fit. The orbital period $P$, the scaled BD radius $r_{\rm BD}=a/R_{\rm BD}$, and the orbital inclination $i$ are derived from the fit of transit light curves. The BD’s surface gravity is related directly to the observable parameters independent of those of the host star.
Figure 15 presents the radius–mass diagram for all known transiting objects with masses ranging 10 and 120 $M_{\rm Jup}$. Figure 16 presents the surface gravity as a function of the mass of known transiting objects. Table 6 shows the updated list of transiting BDs from Carmichael (2022) and Henderson et al. (2024). Some new objects have been included from Vowell et al. (2025). As a preliminary comparison, TOI-6508 b is well placed within the edge of the brown dwarf regime, means, near the hydrogen burning limit (Baraffe et al. 2002). We also presented the tabulated isochrones models for substellar objects derived by Baraffe et al. (2003) (colored solid lines), with different ages of 0.1, 0.5, 1, 5 and 10 Gyr ⁸⁸8Isochrones Models: http://perso.ens-lyon.fr/isabelle.baraffe/.

TOI-6508 b shows a deeper primary eclipse of 250 ppt (parts per thousand), but no detectable secondary eclipse. This implies that the secondary component has a lower surface brightness than the primary component. Phase-folded TESS observations are shown in Figure 17. The absence of a detectable secondary eclipse in the data suggested that the companion is a brown dwarf. Based on TESS data, we might rule out a secondary eclipse of $\delta_{\rm occult}\approx 10$ ppt. We observed two full occultations of TOI-6508 b from LCO-SAAO-1m0 in the Sloan-$i^{\prime}$ on UTC May 16 2024 (assuming a circular orbit) and UTC Feb 8 2025 (assuming an eccentric orbit of $e=0.28$ constrained from our global MCMC analysis). Based on these observations, we might rule out a secondary eclipse of $\delta_{\rm occult}\approx 3$ ppt (see Figure 5). Moreover, the effective temperature $T_{\rm BD}$ for the companion can be computed by combining the companion and stellar radius ratio $R_{\rm BD}/R_{\star}$ with the Planck function for blackbody via the formula:

where $B_{\rm BD}(\lambda,T_{\rm BD})$ and $B_{\star}(\lambda,T_{\rm eff})$ are the Planck distribution functions for the companion and host star, respectively. This resulted in an effective temperature of the companion of $T_{\rm BD}<1800$K, indicative of a brown dwarf. Since the luminosity of a BD is mainly emitted at infrared wavelengths, the secondary eclipse should be deeper when observed at infrared wavelengths. Moreover, a new observation in the infrared is required to confirm or not the secondary eclipse of the companion. If a secondary eclipse is present, this will allow an independent determination of the effective temperature of the brown dwarf TOI-6508 b. Moreover, the combination of low mass and low luminosity of the host star, and low incident flux of the companion, make TOI-6508 b a favorable target for upcoming secondary eclipse observation with the JWST, in order to measure its luminosity, its Albedo, and its effective temperature.

The postdoctoral fellowship of KB is funded by F.R.S.-FNRS grant T.0109.20 and by the Francqui Foundation. This publication benefits from the support of the French Community of Belgium in the context of the FRIA Doctoral Grant awarded to MT. MG is F.R.S.-FNRS Research Director. Author F.J.P acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033 and Ministerio de Ciencia e Innovación through the project PID2022-137241NB-C43. This material is based upon work supported by the National Aeronautics and Space Administration under Agreement No. 80NSSC21K0593 for the program “Alien Earths”. The results reported herein benefited from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate. Based on observations collected at the European Southern Observatory under ESO programme 113.27QV.001. Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR24DA010 with the National Aeronautics and Space Administration. Funding for the TESS mission is provided by NASA’s Science Mission Directorate. KAC acknowledges support from the TESS mission via subaward s3449 from MIT. This paper made use of data collected by the TESS mission, obtained from the Mikulski Archive for Space Telescopes MAST data archive at the Space Telescope Science Institute (STScI). Funding for the TESS mission is provided by the NASA Explorer Program. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. This research has made use of the Exoplanet Follow-up Observation Program (ExoFOP; DOI: 10.26134/ExoFOP5) website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Based on data collected by the SPECULOOS-South Observatory at the ESO Paranal Observatory in Chile. The ULiege’s contribution to SPECULOOS has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) (grant Agreement n^∘ 336480/SPECULOOS), from the Balzan Prize and Francqui Foundations, from the Belgian Scientific Research Foundation (F.R.S.-FNRS; grant n^∘ T.0109.20), from the University of Liege, and from the ARC grant for Concerted Research Actions financed by the Wallonia-Brussels Federation. The Birmingham contribution is in part funded by the European Union’s Horizon 2020 research and innovation programme (grant’s agreement n^∘ 803193/BEBOP), and from the Science and Technology Facilities Council (STFC; grant n^∘ ST/S00193X/1, ST/W000385/1 and ST/Y001710/1). The Cambridge contribution is supported by a grant from the Simons Foundation (PI Queloz, grant number 327127). This work makes use of observations from the LCOGT network. Part of the LCOGT telescope time was granted by NOIRLab through the Mid-Scale Innovations Program (MSIP). MSIP is funded by NSF. Some of the observations in this paper made use of the High-Resolution Imaging instrument Zorro and were obtained under Gemini LLP Proposal Number: GN/S-2021A-LP-105. Zorro was funded by the NASA Exoplanet Exploration Program and built at the NASA Ames Research Center by Steve B. Howell, Nic Scott, Elliott P. Horch, and Emmett Quigley. Zorro was mounted on the Gemini South telescope of the international Gemini Observatory, a program of NSF’s OIR Lab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. on behalf of the Gemini partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).